Structural products with polygonal sections (e.g. rectangular box sections) or such as to present a high moment of inertia (section irons) for achieving a high degree of flexural and torsional rigidity, are commonly used in widely differing fields of industry.
Depending on the type and thickness of the material and the required shape, such products are generally manufactured using standard permanent deformation (pressing, bending) or extrusion techniques.
A drawback common to all the above techniques is the high cost of the equipment involved, which not only affects the cost of the finished product, but also results in a substantially total absence of versatility J at production level. No firm, in fact, would normally contemplate replacing production machinery before it is entirely written off.
In the case of permanent deformation (pressing or bending), in particular, the loads required for achieving marked local deformation of the material (e.g. for deep drawing, forming sharp edges, etc.) invariably result in springback of the material, so that the pressed or bent part springs back to a different shape when the load is removed.
The dimensional inaccuracy resulting from this is especially evident when the part is connected and welded, e.g. spot welded, to another, which generally requires that the two parts be maintained contacting under load. When the load is removed after welding, however, the parts nevertheless tend to relax, thus permanently stressing the weld spots throughout the working life of the structure. Particularly in the case of structures subjected to repeated, impulsive stress (e.g. vehicles), such stress is responsible for sudden, at least macroscopic, failure of the weld spots, which spreads "domino" fashion from one spot to the next along the weld line, at times with catastrophic consequences to the structure.
The above problem, which is partly due to traces of machining oil in the molten material during welding, and which, on vaporizing, result in metallurgical irregularities and microcracks, has to some extent been solved by performing an additional pickling stage prior to welding. Besides only partially solving the problem, however, pickling adds even further expense to the already high direct and indirect cost of conventional methods.
In addition to the oil effect described above, weld spot failure is also caused by electrolytic erosion resulting from an accumulation, in turn aggravated by the capillary effect, of damp, water or dirt in the weld spot area.
Extrusion processes also present numerous drawbacks, foremost of which are high operating and energy costs; invariably poor finish; and serious limitations in terms of the material, size and shape of the finished part.